You can change your surname at your will following the procedures of law for change of name.
There is no restriction on that.
It i a matter of change of name only hence you can proceed with your desire and decision to change your name as per your own will and wish.
Howe4ver since the child is born to both you and your divorced husband, to change the name of your son, you may have to obtain consent or NOC from his biological father.
By your divorce with your husband your son's relationship with his father has not been totally cut or discarded.
Even though you may be given the child custody through court, it is just a matter of custody over the minor child and not that you are entitled to entire rights of your child by yourself.
You cannot change the name of your child from the name appearing in the birth certificate to the name as per your desire at your whims or your wish.
If your ex-husband drags you to court of law in this regard you may have to face the legal consequences.
Hence do not take a hasty decision on this, if at all you want to change the name of your child (even it may be surname), you may better take his consent/NOC or at least issue a legal notice to him for this purpose.